Protective relays are employed to monitor whether fault conditions exist in multiphase electrical lines and to then electrically isolate faulted phases. Some protective relays utilize delta phase selectors to detect the faulted phases in power circuits. Whether the implementation is achieved with analog components or digitized for numerical relay, the process starts by passing the phase voltages or currents through a filter that removes the nominal frequency component of the power system. Under normal conditions, when there is no disturbance or fault event, the output of the change detect filter will be zero. When a fault in the power circuit occurs, a change in the current or voltage waveform will occur and the change detect filter will have a non-zero output. The magnitude of the change is indicative of the significance of the actual change in the power circuit as represented by the voltage and/or current values.
An example of a protective relay that utilizes a delta phase selector includes U.S. Pat. No. 4,409,636 (hereinafter “the '636 patent”) to Brandt et al. which is hereby incorporated by reference. Though these prior art methods have proven effective, not as accurate in their determination of the faulted phase(s), and therefore better accuracy is desirable.